The present invention relates to a new offset printing method and a printing plate for use in a usual light-load printing field, particularly in an offset printing operation, and more particularly to an offset printing method and a printing plate capable of easily making a printing plate. More particularly, the present invention relates to an offset printing method with which a printing plate can repeatedly be reused. The present invention also relates to the above printing plate and a printing apparatus using the above printing method.
The offset printing method has a process for making a printing plate which is very simple among a multiplicity of printing methods. Therefore, the offset printing method has usually been employed as a main printing means at present. The foregoing printing technique is structured to use the immiscibility between oil and water. An oil material, that is, ink is selectively maintained in an image region, while dampening water is selectively maintained in non-image regions. Therefore, direct contact with a surface on which an image will be printed or contact with the same through an intermediate medium, called a “blanket” causes ink in the image portion to be transferred. Thus, printing is performed.
The offset printing operation is mainly performed by a method using a PS plate incorporating a support member which is an aluminum substrate and on which a diazo photosensitive layer has been formed by coating. The PS plate is configured such that the surface of the aluminum substrate, which is the support member, is grained, subjected to anode oxidation and other processes to improve the receiving performance and a repulsion characteristic of the non-image portion against ink. Thus, printing resistance is raised and the printing surface is made to be precise. Then, an image, which must be printed, is formed on the printing surface. Therefore, the offset printing has characteristics including simplicity, printing resistance and precise printing surface.
Since the precise characteristic has been attained to the offset printing method, the offset printing method has widely been used in a usual printing field. On the other hand, the offset printing method has been required to have a furthermore simplified structure. Thus, a variety of simple offset printing methods have been suggested.
A representative method of the simple offset printing method is a method using “Copyrapid” offset printing plate marketed by Agfa-Gevaert Ltd. Moreover, printing methods of the foregoing type have been disclosed in U.S. Pat. No. 3,511,656 and Japanese Patent Publication No. 7-56351A. The disclosed method is arranged to make a printing plate by a silver-salt diffusion transfer method. The foregoing method is able to form an image, which must be transferred, in only one step. Moreover, the image, which must be transferred, has a lipophilic nature. Therefore, the foregoing image can directly be used as the printing plate. Hence it follows that the foregoing printing method has been put into practical used as a simple printing method. However, the foregoing simple method requires a diffusion transfer development step using alkaline developing solution. Therefore, there is a requirement for a simple printing method which does not require a developing step using developing solution.
A method of making a simple printing plate in which the developing step using the alkaline developing solution and arranged to be performed after exposure of an image is omitted has been developed under the foregoing circumstances. In the technical field of the simple printing plate also called an “unprocessed plate” because the developing step can be omitted, a variety of means for forming an image on a recording surface has been suggested which are based on a variety of principles as follows: (1) an irradiated portion of the surface is broken due to heat of exposure; (2) an irradiated portion has a lipophilic nature by the exposure; (3) an irradiated portion is harden due to a light mode to have a lipophilic nature; (4) surface nature is changed due to light-decomposition of diazo compounds; and (5) fusible thermal transfer using a heat mode, and so on.
Disclosed techniques adaptable to the foregoing simple offset printing method include a technique disclosed in U.S. Pat. No. 3,506,779, No. 3,549,733, No. 3.574,657, No. 3,739,033, No. 3,832,948, No. 3,945,318, No. 3,962,513, No. 3,964,389, No. 4,034,183, Nos. 4,081,572, 4,693,958, No. 4,731,317, No. 5,238,778, No. 5,353,705, No. 5,385, 092, No. 5,395,729 and European Patent No. 1068.
Each of the foregoing techniques has been devised such that no developing solution is required when the printing plate is made. Each of the techniques has one or more problems which include insufficient difference between lipophilic regions and hydrophilic region, causing the image quality of the printed image is unsatisfactory, insufficient resolution which a printed image exhibiting excellent sharpness cannot be obtained, insufficient mechanical strength of the image surface and, therefore, occurrence of easy damage of the image surface, deterioration in the simplicity caused from provision of a protective film to prevent the damage and insufficient durability to endure printing for a long time. Therefore, simple omission of the alkali developing step cannot improve the degree of practical use. The requirement for a method of making a printing plate satisfying a variety of characteristics required for a printing operation and capable of simply making a printing plate has not been met in spite of a plurality of the foregoing improvements.
As one of a method of making the unprocessed printing plate, a method of making a printing plate has been disclosed in Japanese Patent Publication No. 9-169098A which uses a fact that zirconia ceramic is caused to have a hydrophilic nature owing to irradiation with light. The photosensitivity of zirconia, however, is insufficient and an unsatisfactory photoconversion effect from the hydrophobic nature to the hydrophilic nature inhibits easy identification between image portions and non-image portions.
If means capable of easily reuse and reusing the used printing plate can be employed in addition to the simple printing method which does not require the developing solution, an advantage can be obtained in that the cost and the quantity of waster can be reduced. When the printing plate is reused, the reusing operation must easily be performed. It is difficult to structure a simple reusing operation. There has substantially no investigation has been performed. Only a special material for the printing plate, which is zirconia ceramic, has been disclosed in Japanese Patent Publication No. 9-169098A.